- Melt the butter in a soup pot or large saucepan over medium heat. Add the leeks, carrot, celery, and a pinch of salt. Cook, stirring often, until very soft but not browned at all, about 12 minutes. Stir in the garam masala, ground ginger, coriander, and cayenne. Cook for 1 minute, stirring continuously.
- Add 3 cups stock, sweet potatoes, apples, thyme, and 2 teaspoons salt. Bring to a boil over medium-high heat, reduce the heat, and simmer until the vegetables and apples are very soft, about 30 minutes. Stir in the cider, grated ginger, and maple syrup. Simmer for 10 minutes.
- Working in batches, purée the soup in a blender and return it to the pot or purée the soup directly in the pot with an immersion blender. If the soup is too thick, add more stock. Taste the soup and add more salt or brown sugar if needed.
- Whisk in the half-and-half. Keep the soup warm over low heat, but don’t let it boil. (Make-ahead note: Cool, cover, and refrigerate for up to three days. Reheat gently and check the seasoning before serving.)
Whisk together the apple butter, crème fraiche, and a tiny pinch of salt in a small bowl. Cover and refrigerate until needed. Whisk before using.
- Gather the ingredients into a small pile on a cutting board and chop until the mixture reaches the texture you prefer, from coarse to finely chopped. Make sure the mixture doesn’t turn into a paste.
- To serve, ladle the soup into serving bowls, top with a little apple butter cream, and sprinkle with a little gremolata. Serve warm.